One problem to be solved in the field of the detection of electromagnetic signals lies in the fact that the type of signal intercepted is not a priori known, particularly its frequency bandwidth, the type of modulation used or, more generally, any parameter associated with the waveform of the signal.
The known detection methods are generally constructed on the a priori knowledge of the form of the signal and use a filter adapted according to this knowledge.
However, it is not possible to implement filters adapted to all types of signals expected.
Two major types of receivers have hitherto been considered to provide a watch over a very wide frequency band: the receivers permanently covering the band to be watched, which are adapted to detect only the signals of high power, and the narrowband receivers, which do not make it possible to instantaneously cover the total band, but whose function is to detect signals of lower power and which allow for finer analyses of the signal.
The present invention falls within the scope of the narrowband receivers.
The traditional methods for detecting electromagnetic signals are notably based on the following preliminary steps.
The reception of signals is done through an antenna array with space diversity, or interferometric array, and the demodulation of the signal is performed by the same local oscillator for all the sensors of the array. The signal is then sampled, on each reception channel, in real or complex form, then one or more banks of filters are applied, for example by weighted discrete Fourier transform. In other words, a number of temporally overlapped discrete Fourier transforms are applied in order to produce an average adaptation to the band of the signals of interest. At the end of this operation, called time-frequency analysis, the signal is transformed into a time-frequency grid broken down into time-frequency cells, each cell containing the result of a discrete Fourier transform for a given time interval and a given frequency interval.
One known detection method consists in comparing the power of the signal, in each time-frequency cell, to a given detection threshold. However, this cell-by-cell decision-making is not optimal when the signal is spread in time and/or in frequency.
There is therefore a problem to be solved in adapting to the spread of the signal over a plurality of time-frequency cells.
Also known are solutions based on the formation of a single channel from the signals received by the plurality of antenna elements of the array, but these solutions are efficient only for regular arrays, in other words arrays for which the distance between two antenna elements is regular.
There is therefore also a problem to be solved in designing a detection method adapted to a lacunary array, that is to say an array for which the spacing between two elements is not regular.